conservation of mass and energy

A principle, resulting from Einstein's special
theory of relativity, which combines the separate laws of conservation
of energy and of mass.
The law of conservation of energy states that in any system energy cannot
be created or destroyed, and the law of conservation of mass (or matter)
states that in any system matter cannot be created or destroyed. These laws
are now seen to be approximations which can only be applied to systems not
involving nuclear reactions or
speeds approaching the speed of light.
The general principle of the conservation of mass and energy, which is of
universal applicability, and which is based upon the mass-energy
relationship.